Anyone who wants a polar bear fur on their skin in uncomfortable winter weather could possibly be helped – with a new combination of materials from a laboratory at Zhejiang University in Hangzhou, China. Researchers there have developed a yarn that resembles polar bear hair, but is not nearly as thick when knitting a sweater with it. The researchers recently reported this in the journal “Science”.
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“The hairs of a polar bear’s fur have a porous core that is covered with a dense shell. This structure keeps the animals warm and dry even at minus 40 degrees Celsius, but is relatively light,” says Bai Hao, one of the authors. The shell is around 20 micrometers thick, which is almost a quarter of the diameter of the bear’s hair.
The porous core of the new yarn is an airgel that consists of 90 percent air and 10 percent chitosan. Chitosan is a well-known bioplastic whose raw material, chitin, can be obtained from shrimp waste. Aerogels themselves are not a new invention and their heat-insulating properties have been known for almost 100 years. They have long been used in space travel and the construction industry. But attempts to use aerogels for textiles have so far failed. According to the study, pure airgel fibers are simply too brittle for knitting or weaving.
Flexible coat around porous shell
The researchers now want to have solved this problem by covering the airgel core with a flexible material based on the polar bear fur model. The choice fell on polyurethane, a thermoplastic with which some rain jackets and trousers are also coated. The researchers optimized the ratio of core diameter and shell thickness – and thus between insulation quality and mechanical flexibility – and were ultimately able to present, among other things, a white knitting yarn that could be woven, knitted and dyed. According to the publication, a fabric made from it can also be washed in the washing machine without any loss of quality.
The team was also satisfied with the mechanical performance of the airgel yarn. The threads could be bent and pulled to many times their length without breaking. And a 0.5 millimeter thick coated airgel thread held an attached 500 gram weight without breaking.
Sweater warms like a down jacket
To measure the heat-insulating properties, the researchers worked with an infrared camera. The images showed: At 40 degrees Celsius, the airgel fabric insulated around 50 percent better than wool and more than twice as well as PET or nylon. A test subject who tried out a knitted sweater made of the material at minus 20 degrees was just as warm in it as in a down jacket that was about five times thicker.
For production, the researchers used established processes similar to those used in the production of other chemical fibers – such as polyester, polyamide or polyacrylonitrile. They created the thread-like airgel core from a polymer solution that was pressed through a spinneret at low temperatures. “By controlling the extrusion speed and the temperature of the cold source, we were able to tune the porous structure within the airgel thread,” they report in the study. The thread was then freeze-dried, coated with a solution containing polyurethane and dried.
So far it is only a feasibility study, the authors admit. But producing the airgel yarn is quite cost-effective. In addition, the fabric made from the yarn is not only suitable for clothing, but also for technical products, such as membranes. In order for their vision to come true, the thread modeled on polar bear fur would now have to be produced in larger quantities.
(Older brother)
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